FIG. 10 is a block diagram showing a control system of a conventional AC servomotor. In this control system, a position feedback value detected by an encoder, etc. is subtracted from a position command to obtain a position deviation, and the obtained position deviation is then multiplied by a position gain in term 1 to obtain a speed command by a position loop control. A speed feedback value is subtracted from the speed command to obtain a speed deviation, and a speed loop process of a proportional-plus-integral control is performed in term 2 to obtain a torque command (current command). Further, a current feedback value is subtracted from the torque command and a current loop process is performed in term 3 to obtain a voltage command of each phase. Based on the voltage commands, the AC servomotor M is controlled by a PWM control, etc.
In controlling a three-phase AC servomotor in the above-mentioned control system, an alternating current control method for controlling currents of three phases individually in a current loop. In this current control method, a torque command (current command) obtained by the speed loop process is multiplied by each of sine waves which are shifted by an electrical angle of 2.pi./3 for U, V and W phases, respectively from a rotor position .theta. of the servomotor detected by the encoder, to obtain a current command of each phase. Then, current deviations are obtained by subtracting actual currents Iu, Iv, Iw detected by current detectors from the three current commands, respectively, and a proportion-plus-integral (PI) control for currents of the individual phases is performed to output command voltages Eu, Ev, Ew for the respective phases to the power amplifier. In the power amplifier, PWM control is performed by an inverter, etc. to provide currents Iu, Iv, Iw for the individual phases to flow in the servomotor M, thus driving the servomotor M. As a result, a current loop is formed as the innermost minor loop of the position and speed loops, and this current loop controls a current flowing in each phase of the AC servomotor.
In the above method for controlling the currents of the three phases separately, since the frequency of each current command rises as the rotational speed of the motor increases to cause the gradual phase lag of the current, the reactive component of current increases to rise a problem that torque cannot be generated with good efficiency. Also, since the controlled variable is alternating current, even in a steady state in which the rotational speed and the load are constant, deviations such as a phase lag with respect to the command, attenuation of the amplitude, etc. occur, making it difficult to attain torque control comparable to that attainable with a direct-current motor.
As a solution to the above problems, a DQ control method is known in which the three-phase current is converted into a two-phase, i.e., D- and Q-phase, in direct-current coordinate system through a DQ conversion, and then the individual phases are controlled by direct-current components.
FIG. 11 illustrates a control system in which an AC servomotor is controlled through the DQ conversion. It is assumed that the D-phase current command is "0", and that the current command for Q-phase is a torque command outputted from the speed loop. In a converter 9 for converting the three-phase current to a two-phase current, D- and Q-phase currents Id and Iq are obtained by using actual currents of u-, v- and w-phases of the motor, and the phase of the rotor detected by a rotor position detector 7, and the currents thus obtained are subtracted from the command values of the respective phases, to obtain D- and Q-phase current deviations. In current controllers 5d and 5q, the respective current deviations are subjected to proportional and integral control, to obtain d- and q-phase command voltages Vd and Vq, respectively. Another converter 8 for converting the two-phase voltage to a three-phase voltage, obtains u-, v- and w-phase command voltages Vu, Vv and Vw from the two-phase command voltages Vd and Vq, and outputs the obtained command voltages to a power amplifier 6, whereby currents Iu, Iv and Iw are fed to the respective phases of the servomotor by means of inverters etc. to control the servomotor.
Generally, the D-phase and Q-phase voltages Vd, Vq converted by DQ conversion can be expressed by the following equation (1): ##EQU1##
Therefore, ##EQU2##
Now assuming that R+sL=Z, the following equations (1)' are obtained. ##EQU3##
Adopting a direct-current control method by DQ conversion, it is possible to reduce a usual deviation without setting a current loop gain in an unnecessarily high level. However, in order to realize the direct-current control method, a large torque is necessary in sudden acceleration at high speed rotation and, therefore, the current command may exceed the limit of the power amplifier to cause a so-called voltage saturation so that the current is difficult to control.
In this case, the value of the integrator of the current loop increases. If the value of this integrator becomes excessively large, a maximum voltage command is kept being outputted for a while, even after the current command becomes smaller, so that an operation of the current loop after the saturation of the voltage command would not be stable.
To cope with this problem, it has been a common practice to perform the following saturation process. FIG. 12 shows D-phase and Q-phase control systems of a conventional AC servomotor, and FIG. 13 shows D-phase and Q-phase command voltages in the saturation process.
In FIG. 12, D-phase and Q-phase controllers are provided with an integral term 11, 12 (K1 is an integral gain) and a proportional term 13, 14 (K2 is a proportional gain), respectively, and the motor is represented by a resistance R and an inductance L. The D-phase and Q-phase controllers are provided with mutual interference terms 15, 16, respectively.
In FIG. 13, assuming that Vc represents a composite command voltage of the D-phase and Q-phase command voltages Vd, Vq and that Vdc represents a DC link voltage which is the maximum output voltage of the power amplifier, the saturation process is performed in the following manner.
(1) The voltage command Vc is outputted as it is, in the relationship Vd.sup.2 +Vq.sup.2 .ltoreq.Vdc.sup.2 (the vector of the composite command voltage Vc is within or on a circle of the DC link voltage).
(2) The D-phase and Q-phase voltages Vd, Vq of the voltage command Vc are clamped in the following values, in the relationship Vd.sup.2 +Vq.sup.2 &gt;Vdc.sup.2 (the vector of the composite command voltage Vc is out of the circle of the DC link voltage). EQU Vd=Vdc.multidot.Vd/(Vd.sup.2 +Vq.sup.2).sup.1/2 ( 2) EQU Vq=Vdc.multidot.Vq/(Vd.sup.2 +Vq.sup.2).sup.1/2 ( 3)
And the values of the integrators are rewritten so that the outputs of the D-phase and Q-phase current controllers are the clamped D-phase and Q-phase voltages Vd, Vq, respectively.
Assuming that k1 represents an integral gain of the current loop, k2 represents a proportional gain of the current loop, I represents a torque command and Ifb represents a current feedback, the voltage command Vc is expressed by the following equation (4). EQU Vc=k1.multidot.(I-Ifb)/s-k2.multidot.Ifb (4)
From the equation (4), the maximum voltage command Vcmax set by the foregoing clamping is expressed by the following equation (5). EQU Vcmax=k1.multidot.(I-Ifb)/s*-K2.multidot.Ifb (5)
The integrator represented by 1/s* is set so that the current controller outputs the maximum voltage command Vcmax, and is expressed by the following equation (6). EQU 1/s*=(Vcmax+k2.multidot.Ifb)/k1 (6)
The integrators for the D-phase and Q-phase current control loops are expressed as follows: EQU D phase: 1/s*=(Vdmax+k2.multidot.dfb)/k1 (6-1) EQU Q phase: 1/s*=(Vqmax+k2.multidot.qfb)/k1 (6-1)
By the saturation process of rewriting the integrators, the current controllers output the clamped D-phase and Q-phase voltages to restrict the composite voltage output Vc always within the DC link voltage Vdc.
However, in the conventional current control method in which the saturation process is performed for both the D and Q phases in saturation of the voltage command, the acceleration characteristic would be lowered.
In the current control using DQ conversion, the D-phase current Id in the same orientation of magnetic flux .phi. is set to "0" and the Q-phase current Iq perpendicular to the D-phase current Id is controlled so as to follow the torque command. Assuming that .omega. represents an angular speed of the rotor, .omega.&gt;0 and Iq&gt;0 when the rotor is rotating forward and is being accelerated. The D-phase and Q-phase voltages at that time is shown in a vector diagram of FIG. 14.
When the composite vector voltage Vc of the D-phase and Q-phase command voltages Vd, Vq exceeds the voltage limit value Vlim (Vdc), the composite vector voltage Vc is converted into a voltage Vc' by changing its magnitude to Vlim with its phase unchanged so that the command voltages are clamped. This relation can be expressed by the following equations. EQU Vd=Vlim.multidot.sin .theta. (7) EQU Vq=Vlim.multidot.cos .theta. (8)
With respect to the phase .theta., there is a relation tan .theta.=Vd/Vq.
FIG. 15a shows the relation between the D-phase command voltage and the current, and FIG. 15b shows the relation between the Q-phase command voltage and the current, when the composite voltage Vc does not exceed the voltage limit value Vlim.
In the current control system by D-Q conversion, the D-phase current command to flow an invalid current is set to "0" and the current control is performed by the Q-phase current command. At that time, as is shown by the equation (1), a negative voltage (-.omega.L.multidot.Iq) is generated in the D phase due to the voltage interference in the motor.
In the case where the composite voltage Vc does not exceed the voltage limit value Vlim, the D-phase command voltage Vd reaches the interference voltage (-.omega.L.multidot.Iq) as shown in FIG. 15a, so that no current flows in the D phase. Therefore, any interference voltage due to the D-phase current does not appear in the Q phase.
When the D-phase and Q-phase command voltages are clamped as the composite command voltage Vc exceeds the voltage limit value Vlim, the D-phase command voltage Vd can not reach the interference voltage (-.omega.L.multidot.Iq), so that a positive current Id flows in the D phase as shown in FIG. 16a. This positive D-phase current Id adds (.omega.L.multidot.Iq) to the Q-phase voltage Vq in FIG. 16b, thus increasing the Q-phase voltage Vq as expressed by the following equation (9): EQU Vq=.omega..phi.+Z.multidot.Iq+.omega.L.multidot.Id (9)
Namely, when the command voltages are clamped by voltage saturation during acceleration, the D-phase voltage Vd is decreased and the Q-phase voltage Vq is increased by the Q-phase current Iq, so that the phase .theta. of the clamped composite voltage Vc decreases to the composite voltage Vc" indicated by a dotted line in FIG. 14. If the phase .theta. is delayed as the D-phase current Id increases, the generated torque decreases. In other words, when the Q-phase voltage is clamped to the DC link voltage Vdc, an adequate voltage for increasing the Q-phase current is difficult to obtain by the increase of the D-phase current Id, thus lowering the acceleration characteristic of the servomotor.
In FIG. 17 showing a relation between the D-phase and Q-phase voltages, when the current flows in the positive direction in the D phase during acceleration, the vector voltage is shifted inwardly of the DC link voltage as indicated by a reference character A, so that the voltage saturation is relieved. Contrary, when the current flows in the negative direction in the D phase, the vector voltage is shifted outwardly of the DC link voltage as indicated by a reference character B, so that the voltage saturation is promoted. As is understood from the equation (1), since the current flows usually in the negative direction in the D phase due to the voltage interference during acceleration, the voltage saturation is promoted.